The present invention relates to a method for reducing the dissolved sulfide content within a wastewater stream and thereby the hydrogen sulfide emissions there from and for improving water quality and treatment plant operations by employing a transition metal salt and an oxidant as additives to the wastewater stream.
Hydrogen sulfide (H2S) is a toxic, corrosive gas that is generated within the biomass adhered to pipe walls and sediment of a sewage system. As wastewater is conveyed through the sewage collection system to the wastewater treatment plant, septic conditions develop that foster the growth of hydrogen sulfide-producing bacteria. Hydrogen sulfide volatilizes from the wastewater into the vapor space of the sewage system where it creates the problems of nuisance odors, infrastructure corrosion, and worker hazards.
The use of iron salts alone to control hydrogen sulfide emissions in wastewater is known in the industry. Iron salts control hydrogen sulfide (H2S) by converting volatile H2S dissolved in the wastewater into nonvolatile iron-complexed sulfide (FeS):
H2S+FeCl2xe2x86x92FeS+2 HCl.
Ferrous sulfide (FeS) is a black precipitate that is stable in the absence of acid and typically settles out in clarifiers at the wastewater treatment plant, where it enters the solids stream. The stoichiometric chemical requirement is 1.7 pounds Fe (or 3.7 pounds FeCl2) per pound H2S controlled, yielding a cost of approximately $0.50 per pound hydrogen sulfide depending on the per-unit chemical cost. Additionally the efficiency of iron salts is not impacted by oxygen uptake rates within the wastewater.
Despite these advantages, the use of iron salts alone to control H2S emissions in wastewater has shortcomings. Iron salts alone lose efficiency when achieving H2S emissions control for more than about four hours hydraulic retention time. Hydraulic retention time is defined as the length of time a component resides within the sewage system. Thus the efficient use of iron salts alone to control H2S emissions of wastewater requires a series of iron salt injection facilities located along the course of the wastewater collection system. At each injection site, the spent iron salt (FeS) is augmented with fresh iron salt. As used herein, the term xe2x80x9ciron saltxe2x80x9d refers to nearly any iron compound (as distinguished from elemental iron) and expressly includes iron hydroxide (Fe(OH)2, Fe(OH)3, FeCl3, FeCl2, FeSO4, and Fe2(SO4)3) but excepts FeS, which is often referred to herein as xe2x80x9cspent iron salt.xe2x80x9d The spent iron salt largely remains inert throughout the treatment and disposal processes. When the wastewater reaches the treatment plant, the mass of spent iron salt settles out in the primary clarifiers. The iron salt demand increases 2-4 fold to achieve H2S emissions control for greater than about four hours hydraulic retention time, thus increasing the amount of spent iron salt generated. The FeS precipitates and constitutes a theoretical solids load of about 3 pounds per pound H2S controlled. The FeS precipitate can cause deposition problems within the sewage system, particularly in low-velocity sewage systems and clarifiers/thickeners as it settles out, thus increasing the actual cost per pound H2S controlled by 20% ($0.075) or more.
Iron salts also degrade the quality of wastewater. The salinity of wastewater is increased by the addition of iron salts, as a minimum of 3 pounds sodium chloride per pound H2S controlled is generated when FeCl2 or FeCl3 is used as the iron salt. Iron salts also deplete the alkalinity of the wastewater stream by consuming a minimum of 3 pounds calcium carbonate per pound H2S controlled. Further, iron salt products typically contain 1-4% mineral acid that further depresses the pH of the wastewater. The reduced alkalinity of the wastewater stream in turn reduces the capturing capacity of iron, thus reducing its ability to control H2S to low levels. Furthermore, the depressed pH of the wastewater encourages volatilization of untreated H2S within the wastewater stream. Additionally, iron salts deplete the wastewater stream of dissolved oxygen by consuming a minimum of 5 pounds dissolved oxygen per pound H2S controlled. Thus, while iron salts are useful in controlling H2S emissions of wastewater, it is desirable to minimize the amount of iron salt added to the wastewater stream to minimize the disadvantages associated with the use of iron salts.
It has been reported that a blend of 1 part ferrous to 2 parts ferric iron provides improved control of H2S emissions from a wastewater stream when compared to either ferrous or ferric iron alone. Such a blend, however, is expensive and is subject to the same disadvantages of iron salts previously stated.
The use of hydrogen peroxide (H2O2) alone to control H2S emissions is also conventional. Like iron salts, H2O2 injection facilities within the sewage system are typically located in series, separated by 1-2 hours hydraulic retention time. The use of hydrogen peroxide alone controls H2S emissions in wastewater by two mechanisms: direct oxidation of H2S to elemental sulfur (I) or prevention of H2S formation by supplying dissolved oxygen (II):
H2S+H2O2xe2x86x92So+2H2Oxe2x80x83xe2x80x83(I)
2H2O2xe2x86x92O2+2H2Oxe2x80x83xe2x80x83(II).
Direct oxidation theoretically requires 1.0 pound H2O2 per pound H2S controlled at a cost of about $0.50 per pound H2S and generates 1.0 pound solids per pound H2S controlled, regardless of H2O2 dose. In contrast, prevention of H2S formation by providing a dissolved oxygen supply theoretically requires 4.0 pounds H2O2 per pound H2S controlled at a cost of $2.00 per pound sulfide. The second mechanism also is adversely affected by environmental factors such as hydraulic retention time and oxygen uptake. Thus, the practical H2O2 requirement can be 2-4 times the theoretical H2O2 requirement when retention time increases by 2-3 hours. Therefore, previous H2O2 applications within the municipal wastewater treatment industry are either targeted at point source H2S control, such as at the headworks to treatment plants, where H2O2 may be applied to the wastewater stream to maximize its most efficient mode as an oxidant, or added as a preventative within the wastewater collection system, at costs exceeding $2.00 per pound H2S controlled.
While the independent use of H2O2 to control H2S emissions by wastewater generates no adverse by-products and advantageously oxygenates the wastewater, it presents several shortcomings. Specifically, the oxidation reaction typically requires 15-30 minutes. In addition, control of H2S emissions at 1-2 hours hydraulic retention time or more is expensive, requiring double the injection stations required by FeCl2 control. Furthermore, the efficiency of H2O2 is adversely affected by high oxygen uptake rates. Hence, a mechanism which provides greater and more efficient H2S emissions control within the wastewater treatment system is desirable.
A process for the conversion of aqueous hydrogen sulfide in geothermal steam employing hydrogen peroxide and iron compounds is taught by U.S. Pat. No. 4,363,215 to Sharp. In the disclosed process, hydrogen sulfide is reacted with hydrogen peroxide, wherein the iron compound serves as a catalyst to accelerate the reaction of hydrogen peroxide with hydrogen sulfide. The iron compound catalyst is added in an amount of from 0.5 to 1.0 parts per million expressed as free metal and thus does not complex with the sulfide. U.S. Pat. No. 4,292,293 to Johnson et al. further discloses the addition of polyanionic dispersants to improve the efficiency of the metallic ion catalyst for the oxidation of sulfide by hydrogen peroxide.
The addition of a combination of a ferric salt and an anionic polymer to a water clarifier is a known development in enhancing solids separation and thereby improving the cost-performance of wastewater treatment plants, though such treatment has not yet been widely employed within the industry.
It is the object of the present invention to provide a cost-effective means for reducing hydrogen sulfide emissions throughout the wastewater collection system as well as the wastewater treatment plant while improving water quality and treatment plant operations.
The present invention provides a novel method for integrating the use of transition metal salts and oxidants, specifically iron salts and hydrogen peroxide, respectively, to achieve reduced dissolved sulfide levels and thereby H2S levels within a sewage system. Iron salts are added to the wastewater stream at the upper reaches of the wastewater collection system prior to H2S volatilization to capture H2S dissolved in the wastewater. The captured sulfide, as ferrous sulfide, is then delivered to one or more points downstream of the iron salt addition where hydrogen peroxide is added to the wastewater stream. The hydrogen peroxide destroys the ferrous sulfide and restores the sulfide-capturing capacity of the iron. At the final regeneration point, for example the wastewater treatment plant, the restored iron salt is used to enhance solids separation and sulfide control in primary clarifiers as well as sulfide and struvite control in anaerobic digesters.